Telegram is the creation of Pavel and Nikolai Durovs. The brothers' previous project was VKontakte, a Russian counterpart of Facebook.
Telegram is the answer to WhatsApp. It started with Pavel Durov's desire. Its code-name was Telegra.ph. He wanted to communicate with his brother in a way that would be free of Russian control. VKontakte was already being monitored. Nikolai designed MTProto, a protocol for mobile devices that became the foundation for Telegram. The brothers worked on the app in secret while the Russian shareholders tried to silence VKontakte.
Telegram's main aspect is its care about user data. It's encrypted in many ways and difficult to reach for any third parties. The founders' main goal was to fight for free speech, which Russian authorities try to limit.
Pavel Durov raised funds for Telegram thanks to Digital Fortress. It was a company he had set up secretly while he was still at VKontakte.
Durov promoted Telegram through his social media. He used his Instagram account to post about his "war" against the Russian government. Durov had a lot of followers, so it was a successful strategy in promoting Telegram.